Interpretive Summary: This paper reports on an improved method for correction to coaxial reflection cells commonly used for measurement of soil moisture. These cells are typically utilized by means of time-domain reflectometry (TDR), however, recent work suggests enhanced accuracies could be achieved if the measurements were conducted via frequency domain systems. As recent research is indicating a need to improve the accuracy of soil moisture measurements in support of other geological sciences such as gaseous emissions, this work examined one observed error. The research resulted in supporting evidence between the theoretical and experimental results as to the cause of the error and lead to the development of a technique by which to correct the system to remove this source of error.

Technical Abstract:
Accurate measurement of moisture content is a prime requirement in hydrological, geophysical, and biogeochemical research as well as for material characterization and process control. Within these areas, accurate measurements of the surface area and bound water content is becoming increasingly important for providing answers to many fundamental questions ranging from characterization of cotton fiber maturity, accurate characterization of gaseous emissions due to soil processes such as nitrification and de-nitrification cycles, and soil water conservation research. Currently, the most accurate soil moisture measurement technology utilizes time-domain-reflectometry, TDR, to characterize the apparent electrical permittivity of the materials under test, such as cotton or soil. While TDR has enjoyed a strong following in the soil-science and geological community, it’s accuracy is still limited to +/-4% even after site specific calibrations. It is recognized that one of the dominant errors that TDR is forced to absorb is due to the technique’s inability to remove the effects of loss term in complex permittivity, common in high surface area soils such as heavy loams and clays. As such, a logical pathway to higher accuracy solutions is a transition from time domain to frequency domain measurements based network analyzer style measurements that will allow for separation of the loss term from the delay term in the complex permittivity measurement. This research examines the cause of an observed experimental error for the coaxial probe utilizing network analyzer measurements which is hypothesized to be due to fringe capacitance. This work provides an experimental and theoretical basis for the cause of the error and provides a technique by which to correct the system to remove this source of error. To test this theory, a Poisson simulation model of a coaxial cell was formulated to examine the magnitude and impact the fringe capacitance has on the measurements. The results of the simulation study confirmed the fringe-capacitance theory as well as quantified the correction term required to remove the error from the measurement. Further physical experimental results confirmed the correction factor providing a strong correlation between the experimental and simulation correction factors required. Thus, the results of this study provides supporting evidence to the underlying hypothesis of fringe-capacitance as an error source as well as a technique to remove the fringe capacitance from the measurement, thereby leading to an enhanced accuracy protocol for the practical application of the developed correction algorithm.